High-frequency resistor



May 3, 1949. H. E. wEBBER HIGH-FREQUENCY RESISTOR 4 Sheets-Sheet 1 Il I Alllundnlnl l .vl/IWI #Il f IIIIWIH HW Original Filed Dec. V6, 1943 INVENTOR HUGH E. WEBBEQ SW7/@M ATTORN EY May 3, 1949. H. E. wEBBr-:R

HIGH-FREQUENCY RESISTOR 4 Sheets-Sheet 2 Original Filed Deo. 6, 1943 INVENTOR HUGH E. WEBBEQ f/ #W 1 ATTORNEY 4 sheds-sheet 5 Original Filed Dec. v6, 1943 MAN.

` INVENTOR HUGH ES. l/l/Esze KW TTORNEY May 3, 1949. H. E. wEBBER' 2,458,793

HIGH-FREQUENCY RESISTOR Original Filed Dec. 6, 1945 4 Sheets-Sheet 4 INV OR HUGH Wemag/a :wf/KM ATTORNEY Patented May 3, 1.949

`HIGrH-FREQUENCY RESISTOR Hugh E. Webber, Hempstead, N. Y., assignor to The Sperry Corporation, a corporation of Dela- Wal'e A Original application December 6, 1943, Serial No.

513,164. Divided and this application December 18, 1944, Serial No. 568,629

An object is to provide a concentric line hot wire holder which may readily be opened for replacement of the hot wire unit.

Another object is to provide a hot wire holder which may readily be adjusted or tuned for measuring high frequency power at different frequencies.

An additional object is to provide a hot wire element suitable for measurement of large amounts of power.

Further, an object of the invention is to provide a broad band wattmeter and a wire holder therefor obviating the necessity for tuning.

Another object is to provide methods for making improved resistive hot wire elements and to provide a machine suitable for making hot wire elements adapted for the measurement of small amounts of power.

Other and further objects and advantages will become apparent as the description proceeds.

This application is a division of my co-pending application, Serial Number 513,164, filed December 6, 1943, for Power measurement.

A better understanding of the invention will be affordedby the following detailed description4 considered in conjunction with the accompanying drawings, in which Fig. 1 is a view partially in section of a microwave wattmeter having a wire holder of the concentric line type represented as partially cut by a plane passing through the axes of both the main line and the lateral shorting stub;

Fig. 2 is a longitudinal section of the apparatus of Fig. 1, represented as cut by a plane 2--2 through the axis of the main line and perpendicular to the plane dened by the axis of the main line and the shorting stub in Fig. 1;

Fig. 3 is a ciruit diagram of a power measuring bridge in which a hot Wire mounted in a concentric line may be employed as one arm of the bridge;

of a wire holder for use in a relatively narrow range of frequencies;

Fig. 5 is a cross-sectional view of a portion of the apparatus of Fig. 4, cut by a plane 5-5 as seen when looking in the direction of the arrows;

Fig. 6 is a corresponding cross-sectional'view of a portion of the apparatus of Fig. 4, cut by a plane 5--6;

14 claims. (c1. coi-c7) '4t Fig. 4 is a view, mainly a longitudinal section,

Fig. 7 is a cross-sectional view of a hot wire holder in which the hot wire element is mounted ata current anti-node;

Fig. 8 is an enlarged detail view of the hot wire unit shown in Fig. 7;

Fig. 9 is a sectional view illustrating a step in the process of manufacture of the hot Wire unit shown in Fig. 8;

Fig. l0 is a longitudinal sectional view of a hot wire unit of the Wollaston wire type designed for measurement of small amounts of power;

Fig. 11 is a perspective view of a machine for preparing and assembling the hot wire unit of Fig. 10, without rupturing the delicate wire lament.

Like reference characters are utilized throughout the drawings to designate like parts..

Fig. 1 illustrates a wire holder and a hot wire .suitable for use in a bridge circuit,.such as shown in Fig. 3. One arm II of the bridge represents the hot wire holder of Fig. l. The bridge shown in Fig. 3 comprises in addition to the hot wire unit II, a compensating resistor I2, a standard bridge arm I3 and a fourth bridge arm I4, consisting of a resistor I5, and a current responsive instrument such as a milliammeter I6 in series. A conventional null-indicating galvanometer I'I and a sensitivity-adjusting rheostat I8 are connected in series between conjugate points I9 and 20 of the bridge to form a conventional diagonal bridge arm. A source of direct current 22 in series with rheostats 23 is connected to the remaining conjugate points 24 and 25 of the bridge. As shown, a switch 26 is connected in series with the power current source 22.

The resistances of the bridge arms are so chosen that at a predetermined temperature of the hot wire in unit II, the bridge Will be in balance and this temperature is usually-the one existing when the maximum safe power is being supplied to the hot wire unit II. For maximum sensitivity, the resistances of the four arms may be made substantially equal. The resistance of the resistor I5 is so chosen that the sum of its resistancel and the resistance of the milliammeter IB will equal the desired resistance for the bridge arm I.

When the resistances of the four arms are made equal the total direct current power input to the bridge may readily be calculated from the measured current input and the resistance of the arms. The direct power consumption in unit I I is then one-fourth the power input. However, by making the resistances of the arms of Il and Il equal regardless of the resistances chosen for the arms I2 and I3, and connecting the milliammeter I6 in series with the resistor |5 to form the arm |4, direct current power consumption in the arm may be calculated directly from the reading of the instrument I6 and the resistance of the arm |4.

The wire holder shown in Fig. 1 comprises a main concentric line 21 consisting of an outer cylinder and an inner rod and a stub line 28 also consisting of an outer conductor and an inner rod. Both lines 21 and 28 are made adjustable in effective electrical length for tuning purposes. The outer conductor of the main line 21 comprises tubular members 29 and 3|. The members 29 and 3| are arranged to be separable but are designed to be fastened together in a xed relation during operation of the apparatus by a threaded sleeve coupling 33.

The inner conductor of the main line 21 comprises a hollow rod 39 secured at the lefthand end of the tubular member 29 by an insulating bushing 4|, a hollow rod 42, threaded to a. solid rccl 43 extending to the righthand end of the tubular member 3| and supported at the righthand end by an insulating bushing 44, and a barretter wire or a Wollaston wire 46 mounted in an evacuated tube 45 to form the unit II with wire terminals 41 and 48 making contact with the tubular rods 39 and 42, respectively.

The bulb 45 of the hot wire element I I is shown with a tip 49 at the side resulting from the evacuation and sealing oi of the bulb. To provide space for the tip 49, the connector sleeve 33 is therefore made of suiiicient diameter and an opening I is left at one side of the tubular member 29 to receive the tip 49.

For varying the electrically effective length of the line 21, a slidable line closer is provided cornprising wipers 52 adapted to make electrical connection with the inner surface of the tubular member 3|, carried by a hollow member 53 which is slidably supported by the rod 43 through insulating bushings 54 and 55. For further supporting the member 53, the rod 43 is provided with a sleeve 56 secured thereto, and for insulating an enlarged portion 51 of the hollow rod 53 from the sleeve 56 a cylinder of insulating material 58 is interposed between the members 55 and 51.

For setting the member 53 in longitudinal position it is secured to a wavelength adjusting sleeve 34 which is in turn adapted to be adjustably secured to the tubular member 3| by means of a screw 36 reaching through a slot 35 in the tubular member 3| with a head 31 inside the tubular member 3| and a securing nut 38 adapted to bear against the outer surface of the wavelength adjusting sleeve 34.

For making liner adjustments in wavelength a micrometer adjusting head is provided, comprising a post 59 secured to the righthand end of the member 53, for example, by means of machine screws as shown; a post 6I integral with the sleeve 34 and a right and lefthand screw 62 threaded into openings in the posts 59, 6I and carrying a knurled adjusting head 63.

For permitting microwave energy to be fed to the line 21, and to pass through the barretter wire 46 from another concentric transmission line (not shown) a conventional concentric line coupling 64 is provided at the lefthand end of the line 21. It will be understood that such a coupling provides for making electrical connection to the outer conductor 29 and the inner conductor 39, from corresponding elements of the supply line (not shown).

For making direct current connections with the barretter wire 46, a coupling unit is provided comprising an inner terminal 66 making electrical contact with the internal rod 43 of the concentric line 21, and an outer screw thread terminal 61 making electrical connection with the outer conductor oi the tubular member 3| through a metallic member 68. As previously mentioned the insulators 44, 54 and 58 are provided to prevent creating a direct-current short circuit between the tubular member 3| and the inner rod 43. In order that a direct-current return path from the lefthand terminal 41 of the barretter wire 46 may be made through the outer conductor of the concentric line without short-circuitlng or producing reflections in the high frequency circuit, the stub line 28 is provided.

The stub line 28 comprises an outer conductor 69, which is electrically connected to the outer conductor members 29 and 3l of the main line 21, and an inner conductor rod 1|, which is electrically connected to the inner conductor member 39 of the main line 21. For tuning out any reflections which might tend to be caused by the presence of the wire 46 in the inner conductor of the line 21, and for making the electrical length of the stub line 28 such as to interpose no impedance in the main line 21, the stub line 28 is so constructed that it may be varied in wavelength in a manner similar to that described for the main line 21. Thus, the line 28 is also provided with spring contact members 12 carried by a member 13 and a longitudinal adjusting sleeve 14 provided with coarse and micrometer adjustments as in the case of the sleeve 34.

It will be understood that when the electrical wavelength of the line 28 is equal to an odd number of quarter-waves of the microwave energy fed to the line 21, a direct connection between the inner and outer conductors of the upper end of the stub line 28 will have no effect on the main line 21. In order to avoid reflections from the righthand end of the line 21 back toward the input end 64 the righthand end of the electrical portion of the line 21 should constitute a short circuit with respect to radio frequency.' This is accomplished by making the conducting members 51 and 56 and the insulating sleeve 58 between them of suiiicient length to form a. by-pass condenser. Therefore, with respect to high frequency, the members 51 and 56 may be considered to be electrically connected.

A direct-current circuit may be traced from the direct-current terminal 66 through the rod 43, the barretter wire terminal 48, the wire 46, the wire terminal 41, the inner conductor 39, inner conductor 1|, the member 13, the contact lingers 12, the outer conductor 69 of the stub line 28, the outer tubular member 29 of the main line 21, the outer tubular member 3| thereof, the metallic fitting 68 and back to the second direct-current terminal 61.

For the purpose of compensating the bridge of Fig. 3 for variations in ambient temperature,

which would tend to vary the resistance of the wire 46 for a predetermined power dissipation, a compensating resistor may be mounted in close proximity to the wire 46 so as to be subjected to substantially the same ambient temperature. This resistor may be one of the arms of the bridge, for example, the arm I2 as previously mentioned.

For mounting the resistor I2 in close proximity 'ing retaining discs or flanges 15 formed in a sleeve 16 and in the coupling sleeve 33. Resistance wire of suitable size is wound between the flanges 15 to produce a total resistance of the value desired for the arm I2 of the bridge. Since the wire of the compensating resistor is not protected by enclosure in an evacuated bulb, it will be understood that the wire will need to bei relatively heavy and therefore made of considerable length in comparison with the length and thickness of the wire 46.

When a power measurement is to be made, before the microwave energy is supplied to the coupling 64, the bridge of Fig. 3 is balanced by vadjustment of the rheostats 23 until the current flowing through the barretter wire 46 is such as to produce a standard resistance of the bridge and causing null indication of the galvanometer I1. The milliammeter I6 is then read to ascertain the direct-current power consumption of the arm |I or I4. Thereupon the direct-current power supply to the bridge is reduced to prevent overloading and burning out of the wire 46, and microwave power is supplied to the wire 46 through the connection 64. The rheostats 23 are again adjusted until a balance is obtained by the null indication of the galvanometer I1. The milliammeter I6 is again read and the value of the alternating power input is then determined by the difference between the direct power-inputs to the wire 46 before and after admission of the` alternating current power.

When relatively little frequency variation in the microwave input is to be expected, the longitudinal adjustment of the main line 21 is not needed and the stub line 28 is utilized only for making a direct current return connection from the lefthand terminal 41 of the barretter wire 46. For example, as shown in Fig. 4, the wire holder may take the form of a concentric line unit having a main line 21 and stub line 28. The main line 21 comprises an outer cylinder 11, the lefthand inner rod 39 and the barretter wire unit I I; the stub line 28 comprises an outer cylinder 18 and an inner rod 19. A longitudinally slidable, adjustable-ratio impedance-matching transformer or matching sleeve 80 may be provided.

For thepurpose of shorting the righthand end of the main line 21 with respect to alternating current a by-pass condenser 8| is formed at the righthand end cf the main line 21. The by-pass condenser 8| comprises a metallic fitting 82 with a tapered or conical outer surface and a second fitting 83 with a tapered inner surface mating the outer surface of the fitting 82, but separated therefrom by insulating sheet material 84 tol form a condenser. The fitting 82 is electrically connected to the terminal 48 of the wire 46 by a. suitable means such as by means of an abutting screw 85, and the fitting 83 has a hollow portion 88, adapted to receive a conventional direct current coupling and a neck portion 81 tting inside and making electrical contact with the righthand end of the outer conductor member 11. The direct current coupling 65 includes an outer threaded terminal 61,`making electrical-contact with the member. 83, and an inner terminal 66' connected by aconductor 88 to the fitting 82.

For adjusting electrical length of the stub line 28 the inner surface of the cylinder 18 is threaded and a threaded nut or plug 89 is provided which carries contact fingers 9|, engaging the inner conductor rod 19. A closing cap 92 may be provided. It will be understood that the plug 89 is provided with recesses (not shown) whereby a suitabletool may be utilized for rotating the plug 89 in the threaded inner surface of the cylinder 18.

In this case the direct-current electrical circuit from the inner connector terminal 66' takes place through the conductor 88, the fitting 82, the barretter wire terminal 48, the barretter wire 48, its terminal 41, the inner conductor 39, the inner conductor 19, contacts 9|, plug 89, cylinder 18 and the outer cylinder 11 back to the neck portion 81 of the fitting 83 and the threaded terminal 61'.

To permit mounting and replacing'the barretter wire unit Il, the fitting 83 is made removable, the inner conductor 39 is made hollow at the end-and it is provided with a set screw 93 for receiving and fastening the terminal 41 of the Wollaston wire 46. An opening (not shown) is provided in the cylinder 11 to provide access to the screw 93 by means of a screwdriver. To provide space for the tip 49 of the bulb 45, a channel 94 (Fig. 5) is provided in the side of the hollow cylinder 11, and the sides of the slots 94 are provided with longitudinal ribs 95. For securing the tting 83' to the righthand end of the hollow cylinder 11 a clamp (Fig. 6) is provided comprising a split ring 96 secured to clamping jaws 91 cooperating with a clamping screw 98.

When broad band characteristics are required for a wattmeter, a wire holder may be employed in which the barretter wire is mounted at the current anti-node or loop in a. concentric transmission line and this is accomplished by making the operative portion of the barretter wire substantially less than the operative wavelength.

vAs illustrated in Fig. '7 the wire holder comprises `a concentricr transmission line I0| and a lateral stub |02 with a conventional input connection |03 at the lefthand end of the line for receiving power from a concentric line (not shown), and a. direct current connector |04 at the righthand end.'

The main line I0| comprises a hollow-cylindrical outer conductor |05, an inner conductor rod |06 and a .barretter wire unit |01. The lateral stub |02 comprises a hollow-cylindrical outer conductor |08 joined to the cylinder |05 and a center rod |09. For electrically insulating the righthand end of the hot wire unit |01 from the outer conductor |05, with respect to direct current while maintaining a short circuit at the righthand end of the line I0| with respect to.

|.I6 for making electrical contact with the righty hand end of the barretter wire unit |01.

The barretter wire unit |01, which will be described in greater detail hereinafter in connection with Fig. 8, includes a lefthand terminal I I1 fitting into a suitable axial opening in the rod |08 and a, righthand terminal or metallic end plate I I8 adapted to fit against the flange |I6 of the condenser member II2. The direct current connector |04 includes an inner prong terminal II9 and an outer threaded termina1 |2I. For securing the outer threaded terminal I2I in the righthand end of the hollow cylinder |05 and making electrical connection therebetween a metallic bushing |22 is provided. A pierced insulating plug |23 is provided for supporting the prong ||9 within the outer threaded terminal |2 l.

For holding the plate |8 of the wire |01 against the ange ||6 in the by-pass condenser and for making an electrical connection between the connector prong ||9 and the end plate ||8, a spring unit is provided comprising a tube |24 provided with internal flange |26 at the lefthand end, secured at the righthand end to the insulator plug |23 and containing a compression spring |21, and a spring-pressed contact pin |28 protruding the opening at the lefthand end of 'the tube |24 and having a head |28 abutting the spring |21 within the tube |24.

For terminating the stub line |02, a. wave trap |3| is provided, comprising a pair of concentric hollow cylinders |32 and |33, internally and externally spaced from, but electrically and mechanically connected to a hollow cylindrical extension |34 of a hollow longitudinally slidable plug |35 fitting the inner surface of the hollow cylinder |08. As shown, the hollow cylinders |32 and |33 are electrically connected to the hollow cylinder |34 at the lower end thereof by means of an annular member |36. The hollow cylinders |32 and |33 are spaced from the inner conductor rod |09 and the outer conductor hollow cylinder |08 as well as from the hollow cylinder extension |34, and are spaced also from shoulders |31 in the plug |35. The cylinders |32 and |33 are made substantially one-quarter wave in length for the average wavelength of microwave frequency to be measured or one-half wavelength from the radio frequency end (over one-quarter wavelength section and down to bottom or dead short again) in order to provide a complete termination at the lower end of the plug |35.

To permit adjustment of the electrical length of the stub line |02 in terms of wavelengths, the hollow cylinder |08 may be provided with slots |38, and screws |39 cooperating with the slots |38 and threaded into plug |35 may be provided. If the proper length of the stub line |02 is determined beforehand, it will not be necessary to provide such adjustment, and the barretter wire unit |01 will absorb microwave energy over a range of frequencies extending 40% above and below the mid band frequency for which the apparatus is designed without producing any reflections. As shown, the barretter wire unit |01 is entirely symmetrical with respect to the longitudinal axis through its terminal rod ||1, has no laterally extending sealing tip and utilizes a flat end plate terminal ||8. This shape and structure facilitates the mounting arrangements, permits the simplification of the construction of the outer cylinder and also makes it possible to connect the righthand-end of the power-absorbing wire of the unit |01 directly at the current anti-node.

As shown in greater detail in Fig. 8, the unit |01 comprises a cylindrical glass bulb |4| closed at the righthand end by a metallic end plate I8, having a sealed metallic tube |42 coaxially joined thereto at the lefthand end, and containing a barretter wire |43. For completing the electrical connections between the lefthand rod terminal ||1 and the end plate ||8 through the barretter wire |43, the wire |43 is provided with terminal lugs |44 and |45 welded to the ends of the wire |43 and in turn welded or soldered to the tube |42 and the end plate ||8, respectively.

The glass of which the tube |4| is composed,

and the metal of which the end plate ||6 and the tube |42 are composed are preferably so chosen as to have the same temperature coemcient of expansion. For example, relatively low temperature coemcient of expansion material may be employed such as an alloy known as Kovar alloy, for the elements ||8 and |42, and a glass such as that known as 9X or '105 glass may be employed for the tube |4|.

The barretter element |01 may be prepared in the following manner. Referring to Fig. 9, a metallic tube |46 is prepared by banding it at the righthand end with a globule of glass |41 and sealing the lefthand end to a tube |48 adapted to be connected to an evacuating pump (not shown). A length of wire |43 of suitable material such as one-mil tungsten wire, for example, is secured to the lugs |44 and |46, the lug |44 in turn is secured to the righthand end of the tubular stock |46 and the glass tube |4| is then slipped over the wire |43 and the righthand end of the tubular stock |46. Thereupon the lug |46 is secured to the end plate I8 and the righthand end of the glass tube |4| is sealed to the abutting surface of the end plate I8, which is in the shape of a fiat disc. Thereupon a flame is applied to the lefthand end of the glass tube |4| and the glass globule |41 until a seal |43 is formed as shown in Fig. 8. Thereupon the evacuating pump is sarted and the space within the tubes |4| and |46 is evacuated to a pressure of several microns of mercury. Hydrogen or an inert gas may be introduced if the wire |43 is intended to absorb relatively large amounts of power. Otherwise, a vacuum is retained within the tube |4 Thereupon a llame is applied to the point |5| in the tubular stock |46 until the metal softens sumciently to permit drawing out whereby the stem |1 (shown in Fig. 8) is formed. It will be understood that the vacuum is not cut of! the tube |48 until the opening in the stem ||1 has been closed.

When extremely low power at microwave frequency is to be measured, it is necessary to employ a barretter such as a Wollaston wire unit in which the Wollaston wire is extremely ne in order to be sensitive and it is necessary to employ a mounting capsule therefor which has low loss characteristics. A suitable form of Wollaston wire unit for low power measurements is illustrated in Fig. 10. The unit comprises a capsule consisting of a hollow cylinder |6| composed of low loss insulating material and a pair of brass end plugs |62 threaded into the tube |6|. A plastic material, for example, such as polystyrene may be employed for the tube |6I. In order to insure a strong joint Without danger of stripping the threads in the polystyrene material, relatively fine threads |63 are employed. The plugs |62 are drilled to receive terminal rods |64. The Wollaston wire itself consists of a length of ne platinum wire |65 with a silver coating |66 for strength and with a center loop |61 having the silver coating etched away so that the microwave energy is actually dissipated in the center loop portion |61. The silver coated ends |66 are electrically connected as by spot welding or soldering to inner tips |68 of the terminal rods or pins |64.

The Wollaston wire of the desired fineness is prepared by drawing down silver-coated platinum wire. For low power measurements where platinum wire with a diameter of .00005 inch may be desired, the bare platinum wire is so fragile that great care must be taken in constructing the Wollaston wire unit and mounting the wire in the capsule.

For this purpose a jig such as in Fig. 11 may be employed. The jig comprises a base plate I1| carrying a stationary rod-holding post |12, a longitudinally adjustable post |13 and a third adjustable support |14. The posts |12 and |13 carry removably secured rods |15, which in turn carry pin chucks |16.

Preparatory to forming a Wollaston wire capsule, terminal pins |64 are inserted in the pin chucks |16. Brass end plugs |62 are slipped over the pins .|64 and one of the plugs |62, for example, that shown at the left in Fig. 11, has an insulating tube |6| threaded onto it and also surrounding the terminal pin |64. The plugs |62 and the tube |6| are pushed back so that the inwardly extending tips |66 of the terminal pins |64 are exposed. Thereupon a length of silvercoated platinum Wire |65, which has previously been drawn down to the desired size, is lsecured to the tips |68.

For etching away the silver coating at the center portion of the Wollaston wire |65 in order to expose the bare loop |61 (Fig. 10), a platinum inoculating loop |11 is provided containing a droplet of suitable etching solution such as nitric acid.

The inoculaiing loop |11 is secured to the support |14, which is also provided with adjustments including a vertical adjustment controlled by a micrometer screw |18 and preferably also a longitudinal adjustment controlled by a micrometer screw |19. The inoculating loop |11 has a shaft |8| secured to a movable head |82 of the adjustable support |14.

After the silver coated Wollaston wire |65 has been soldered or welded to the terminal pin tips |68, and before the inoculating loop |11 is brought into proximity, the longitudinal position of the adjustable post |13 is set by means of a micrometer screw |83 so that the wire |65 forms a slightly sagging curve of desired radius. Thereupon the inoculating loop |11 containing the droplet of nitric acid is moved toward the sagging wire |65 from below until contact has been made and the sagging portion of the wire is immersed in nitric acid. The diameter of the inoculating loop vmay be approximately two millimeters, for example. For hastening the etching a microscope |90 having a microscope lamp may be provided which is so mounted as to focus the'beam of the microscope lamp on the immersed portion of the wire in the inoculating loop I 11, thus raising the ternperature of the acid.

In order to prevent the surface tension of the acid from pulling the ne wire apart While the inoculating loop is being withdrawn after the etching has been completed, the drop is preferably slowly lowered by means of the micrometer screw 18. In order to hasten the process and in order to guard'against breakage, the acid may be replaced partially by a droplet of low-surfacetension solvent such as ether, before the inoculating loop |11 is completely lowered. Thereafter, the wire is rinsed with ether and later dilute ammonia to neutralize acid, and the insulating tube |6| may be slipped overthe etched Wire and threaded to the remaining end plug |62 at the right. The plugs |62 are then soldered to the terminal pins |64, as shown at |84, in Fig. l0, care being taken to avoid melting the polystyrene tube |6| during this process. If more or less of the silver coating is to be etched the size of the inocu-v latng loop may be changed or the loop may be l0 moved back land forth along the wire by the micrometer screw |19.

If desired the resistance of the exposed portion |61 of the Wire may be checked as the etching is going on by connecting the wire to a suitable resistance-measuring device such as a Wheatstone bridge |86. The bridge |86 is a conventional four-armed bridge including a. null galvanometer 81 and having the wire |66 connected as a fourth arm of the bridge by means of suitable terminal clips |88 which may be connected to or snapped over the pin chucks |16. The bridge |86 is soadjusted that the galvanometer |81 will reach balance when the Wire |81 has increased to the desired resistance, making due allowance for the eiect of the droplet of nitric acid in the loop |11.

As many changes could be made in the above construction and many apparently widely di'erent embodiments of this invention could be made without departing from the scope thereof, it is in'- tended that all matter contained in the above description or shown in the accompanying drawings .shall be interpreted as illustrative and not in a limiting sense. Y

What is claimed is:

1. A power-absorbing wire unit for a microwave wattmeter, comprising an insulating tube, a power-absorbing wire therein extending longitudinally thereof, a conducting end plate electrically connected to one end of the wire, and a conducting tube extending longitudinally from the opposite end of the insulating tube and electrically connected to the other end of the wire. whereby the unit has a flat terminal at one end and an extending pin terminal at the other end.

2. The method of making a power-absorbing wire unit for a microwave wattmeter, comprising the steps of uniting a wire to a conducting tube, surrounding the joint of said wire and the connected conducting tube with. an insulating tube, uniting the opposite end of the wire with a conducting disc plate, uniting the said plate to one end of the insulating tube, uniting the remaining end of the insulating tube to the conducting tube, evacuating the bulb so formed through the conducting tube and sealing 01T the ing the ends of the insulating tube to the plate.

and the rod terminal respectively to form a wireenclosing bulb.

5'. The method of claim 4, wherein the rod terminal is hollow, and the additional steps are performed of evacuating the bulb through the hollow rod and sealing oil' the hollow rod.

6. A method of preparing an axially symmetrical evacuated power-absorbing unit, comprising the steps of forming a'bulb having an insulated body with a conducting nat plate terminal at one end and longitudinally extending conducting tube terminal at the other end, uniting a wire to the conducting terminals, and evacuatlng the bulb through the conducting tubing. A

7. A power-absorbing unit for an alternating `current bridge, comprising an insulating tube,

connecting plugs at the ends thereof closing the tube, pin terminals extending through said plugs and a power-absorbing wire unit constituting a substantially pure resistive element at the operating electrical wavelengths connected between said pin terminals, said wire unit having a central operative portion of reduced diameter.

8. A low-loss power-absorbing unit for a micro wave wattmeter, comprising in combination, an insulating tube, a pair of metallic plugs threaded into and closing the ends of the tube, pin terminals extending axially through the plugs and fixed thereto, and a power-absorbing wire electrically connected between the inner ends of the pin terminals.

9. The method of making a power-absorbing unit for a microwave wattmeter which comprises the steps of supporting a pair of terminal-forming pins substantially in line with each other, with a space between the adjacent ends, sliding a closing plug over one pin, sliding over the other pin a closing plug having an insulating tube attached thereto coaxially with the pin, attaching a length of power absorbing wire between the adjacent ends of the terminal pins, sliding the insulating tube and its attached plug axially to cover the power-absorbing wire, closing the open end of the tube by inserting the mst-mentioned plug, and electrically securing the plugs to the pins.

10. A power absorbing unit for a high-irequency watt-meter, comprising an enclosed tube, connecting plugs closing the ends of said tube, pin terminals protruding from the inner side of said plugs, and a power absorbing wire constituting a substantially pure resistive element at the operating electrical wavelengths connected between said pin terminals, said wire unit having an operative portion of reduced diameter.

11. A barretter unit for a high frequency transmission line, comprising an insulating tube, connecting plugs attached to the end portions thereof and being provided with interior substantially aligned current conducting projections, and a power absorbing wire within said insulating tube and connected at its ends to said projections for conveying current therebetween, said wire being designed so as to provide substantially pure resistance at the operating electrical wavelengths.v

12. A barretter unit for a high-frequency coaxial transmission line, comprising a cylindrical dielectrical tube, cylindrically shaped connecting plugs attached to said tube, said plugs having a diameter substantially equal to said tube diameter and provided with interior cylndrically protruding terminal pins, a substantially pure resistive high-frequency power-absorbing illament electrically connected to and longitudinally supported by said terminal pins, said illament having an operative portion of its length of reduced diameter, and the operative portion of said filament being substantially less than the operating electrical wavelength.

13. A power-absorbing unit for a high-frequency watt-meter, comprising a cylindrical dielectric tube, cyllndrically shaped conducting plugs attached to said tube and having a diameter substantially equal to said tube diameter and provided with cylindrically protruding terminal pins, a substantially pure resistive high-frequency power-absorbing mament connected to said terminal pins, said filament having an operative portion of its length of reduced diameter.

14. A power-absorbing wire unit for a microwave watt-meter, comprising an insulating tube, a power-absorbing wire therein extending longitudinally thereof, a conducting disc-shaped plate electrically connected to one end of said wire, and conductive closing means extending longitudinally from the end of said insulating tube and electrically connected to the other end of said wire. HUGH E. WEBBER.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number Name Date 623,730 McCutcheon Aug. 25, 1899 794,459 Hogg July 11, 1905 923,797 McNeill June 1, 1909 1,506,852 Morrison Sept. 2, 1924 1,531,955 Kingman Mar. 31, 1925 1,683,067 Dubllier ---4 Sept. 4, 1928 1,740,391 Campbell Dec. 17, 1929 1,862,014 Grondahl June 7, 1932 2,144,888 Mever et al Jan. 24, 1939 2,185,025 Elenbaas et al. Dec. 26, 1939 2,203,545 Peterson June 4, 1940 2,233,546 Meulendyke Mar. 4, 1941 2,265,821 Siegel Dec. 9, 1941 2,359,302 Curtis Oct. 3, 1944 FOREIGN PATENTS Number Country Date 222,564 Great Britain Oct. 2, 1924 557,541

Great Britain Nov. 24, 1943 

